Crystalline thermoplastics such as polyacetals are well known in the art, and are in widespread commercial use, principally in the form of polyoxymethylene (POM) homopolymer and copolymers. Polyoxymethylene homopolymer is generally formed by polymerizing formaldehyde or trioxane, the cyclic ether form of formaldehyde. Copolymers are generally formed by combining formaldehyde or trioxane with various cyclic ethers such as ethylene oxide or 1,3-dioxolane to form a polymer chain having some units with two or more methylene groups directly adjacent to one another, thereby improving the thermal stability over that of the homopolymer. See for example, U.S. Pat. No. 2,768,994, and U.S. Pat. No. 3,027,352.
Polymer-polymer blends of the prior art often comprise soft or elastomeric materials with polyacetal to provide improved toughness. Sometimes this improved toughness is manifested by an increase in resistance to a notched impact, sometimes by an improvement in elongation or falling weight impact, and sometimes by an increased ability for the molded article to be bent without breaking. In order for the soft elastomeric material to toughen the polymer matrix, it must first be dispersed well and into small particles. The small particles must be maintained as small and discrete particles during any subsequent melt processing, such as injection molding. Finally, there must be sufficient adhesion in the solid state to allow energy from an impact or otherwise distorting event to cross the interface from the matrix to the elastomeric particles for them to toughen the part.
Another problem that can occur in the prior art polymer-polymer blends is delamination. Delamination can occur due to the large particles with too little adhesion to the matrix polymer aligning and then fusing into micro sheets of polymer in the high shear regions of the article. These shear regions are typically located just below the surface and immediately downstream from the gate of the injection molded article.
“Compatibility” is an old and broadly used term in polymer blend technology. The meaning of compatibility is often defined by the context in which it is used. Hence, compatibility can vary in meaning from compatible to the naked eye to chemically compatible in that one ingredient will not degrade the other such as would be the case with polyacetal and PVC, to being miscible at the molecular level. For purposes of this application, the term “better or improved compatibility” describes how a polymer blend helps improve toughness or decrease delamination and/or prevent mold deposits from the second phase from being separated out from the matrix and being left on the mold in the form of a deposit after a number of mold shots, the second phase being an elastomer.
Manufacturers and customers of crystalline thermoplastics such as polyacetal compositions and the articles made from these compositions are interested in lower cost and/or improved toughness associated therewith. Thus, it is desirable to provide a crystalline thermoplastic composition (e.g. polymer blends) with improved compatibility, as defined above, with economical polar olefins or using less thermoplastic polyurethanes (TPUs), which are relatively expensive. This improved compatibility including improved or maintained toughness or decreased delamination when applicable, and/or prevention of mold deposits.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
WO 02/14429 to Weinberg et al. discloses a composition and method for forming a polyacetal composition of molded articles that exhibit desirable combinations of stiffness and impact resistance. The composition contains a mineral filler within a polyoxymethylene matrix but does not contain a second polymer or elastomer phase.
U.S. Ser. No. 09/641,149 to Weinberg et al. discloses a composition and method for forming a polyamide composition of molded articles that exhibit desirable combinations of stiffness and impact resistance. The composition contains a mineral filler within a nylon matrix but does not contain a second polymer or elastomer phase.
U.S. Pat. No. 4,521,488 to Hattori et al. discloses a polyacetal resin composition with heat stability and surface processability which comprises (a) about 100 parts by weight of polyacetal resin, (b) about 2 to about 35 parts by weight of a carbonate, phosphate or acetate of a metal belonging to Group II of the Periodic Table or a mixture thereof and (c) about 0.01 to about 20 parts by weight of a polymer, a copolymer or a mixture thereof of a compound selected from the group consisting of unsaturated polyesters, alkyl esters of acrylic acid or methacrylic acid, amides of acrylic acid or methacrylic acid, triallyl cyanurate, diallyl phthalate, vinyl acetate and divinylbenzene.
U.S. Pat. No. 4,753,980 to Deyrup discloses toughened thermoplastic polyester molding compositions characterized by extraordinary toughness comprising 60-97 weight % of a polyester matrix resin and 3-40 weight % of an ethylene copolymer such as ethylene/methylacrylate/glycidyl methacrylate.
U.S. Pat. No. 5,641,824 to Forschirm discloses a self-lubricating polymeric composition, characterized as a melt blend of a thermoplastic polymer and a lubricating system containing ultra high molecular weight polyethylenes, a polyester, acid metal salts, calcium salts, antioxidants and stabilizers. The composition may be prepared into shaped articles, which exhibit improved surface wear resistance and coefficients of friction.